Catalyst and process for preparing color-reduced...

Details Catalyst and process for preparing color-reduced... Catalyst and process for preparing color-reduced...

2000-09-18

2003-04-22

Rotman, Alan L. (Department: 1625)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From reactant having at least one -n=c=x group as well as...

C528S073000, C502S164000, C521S118000

Reexamination Certificate

active

06552154

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalyst and to a process for preparing color-reduced polyisocyanates containing isocyanurate groups.
2. Discussion of the Background
For high-quality one- and two-component polyurethane coating materials possessing good light and weathering stability, polyisocyanate mixtures containing isocyanurate groups and uretdione groups are typically used as the isocyanate component.
For the preparation of polyisocyanates containing isocyanurate groups and uretdione groups, which are suitable as raw materials or polyurethane coating formulations, a variety of processes are known. These processes typically differ in the selection of the trimerization catalysts or in the selection of the organic isocyanates to be used in the oligomerization reaction (cf., e.g., GB 1 391 066, EP-A-0 082 987, DE-A 39 02 078, EP-A-0 339 396, EP-A-0 224 165).
Suitable isocyanates used for trimerization, examples of which include aromatic, cycloaliphatic and aliphatic polyisocyanates having a functionality of two or more, can be prepared by various kinds of processes (Annalen der Chemie 562 (1949) pages 75 ff.). Those which have proven particularly suitable in industry include preparation by phosgenation of organic polyamines to the corresponding polycarbamoyl chlorides and the thermal cleavage of the chlorides into organic polyisocyanates and hydrogen chloride. Alternatively, organic polyisocyanates can be prepared without the use of phosgene, i.e., by phosgene-free processes. According to EP-A-0 126 299 (U.S. Pat. No. 4,596,678), EP-A-126 300 (U.S. Pat. No. 4,596,679) and EP-A-355 443 (U.S. Pat. No. 5,087,739), for example, (cyclo)aliphatic diisocyanates—such as 1,6-hexa-methylenediisocyanate (HDI) and/or isomeric aliphatic diisocyanates having 6 carbon atoms in the alkylene radical, and 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (isophorone diisocyanate or IPDI)—can be prepared by reacting (cyclo)aliphatic diamines with urea and alcohols to give (cyclo)aliphatic biscarbamic esters and thermally cleaving these esters into the corresponding diisocyanates and alcohols. The synthesis takes place continuously in a circulation process and in the presence, if desired, of N-unsubstituted carbamic esters, dialkyl carbonates, and other byproducts returned from the reaction process.
Examples of catalysts which can be used for the trimerization of isocyanates to give the desired polyisocyanates containing isocyanurate groups and uretdione groups include tertiary amines, phosphines, alkali metal phenoxides, aminosilanes, quaternary ammonium hydroxides, and quaternary ammonium carbonates. Highly suitable oligomerization catalysts are hydroxides, halides or carboxylates of hydroxyalkylammonium ions (cf., e.g. EP-A-0 351 873, U.S. Pat. No. 5,290,902), alkali metal salts, and tin salts, zinc salts and lead salts of alkylcarboxylic acids. Depending on the catalyst, it is also possible to use various cocatalysts such as, for example, OH-functionalized compounds or Mannich bases comprising secondary amines and aldehydes and/or ketones.
For the oligomerization, the (cyclo)aliphatic diisocyanates are reacted in the presence of the catalyst, with or without the use as solvents and/or auxiliaries, until the desired conversion is attained. Partial trimerization is one of the terms used in this context, since the target conversion is generally well below 100%. Subsequently, the reaction is terminated by deactivating the catalyst and the excess monomeric diisocyanate is usually separated off, generally by flash distillation or thin-film distillation. Deactivation is carried out thermally or by adding a catalyst inhibitor such as, for example, p-toluenesulfonic acid or bis(2-ethylhexyl) phosphate. Particularly advantageous in the context of the trimerization of isocycanates on the industrial scale is the use of quaternary hydroxyalkylammonium carboxylates as oligomerization catalysts. These catalysts of the choline type are thermally unstable. It is unnecessary to terminate the trimerization on reaching the desired conversion by adding catalyst inhibitors which have the potential to reduce the quality. Instead, the controlled thermal deactivation permits optimum process control. The thermal instability is also advantageous from the standpoint of process safety. Uncontrolled “runaway” of the reaction is impossible, provided the amount of catalyst metered in remains below a multiple of the usual amount.
Depending on the type of catalyst used and the reaction temperature, the resulting polyisocyanates have different proportions of isocyanurate groups and/or uretdione groups. The products are usually clear, although products with a more or less strong yellow coloration may also be obtained depending on the type of catalyst, quality of diisocyanate, temperature of reaction and reaction regime. For the preparation of high-quality polyurethane coating materials, however, products having a very low color number are desired.
The unwanted yellow also occurs when the otherwise highly advantageous quaternary hydroxyalkylammonium carboxylates are used (vide supra), so that there is a specific requirement for improvement in this context. Surprisingly, it has now been found that, in comparison to other catalysts of this type, specific quaternary hydroxyalkylammonium carboxylates provide polyisocyanates, which contain isocyanurate groups, having markedly improved color quality.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for preparing polyisocyates that avoids the problems described above.
This and other objects of the invention have been achieved by the present invention, the first embodiment of which provides a process for preparing color-reduced polyisocyanates containing isocyanurate groups, which process includes:
trimerizing at least one diisocyanate in the presence of 0.04-2% by weight, based on the weight of the diisocyanate, at least one trimerization catalyst of the formula (I):
 wherein
 and wherein:
A, B, C, D and E independently of one another or simultaneously are hydrogen, chloro, C1-C4 alkyl, C1-C4 alkoxy, C1-C4 alkoxycarbonyl, hydroxyl, (R
5
)
3
SiO—, (R
5
)
2
N—, —COOH, (R
5
)
2
N—CH
2
— or phenyl, it being possible for any two adjacent A, B, C, D and E radicals to form a conjoint 5- or 6-membered saturated or unsaturated ring which may optionally and additionally include N, S or O as heteroatom;
F is hydrogen or methyl;
R
2
and R
3
independently of one another or simultaneously are C1-C4 alkyl, C2-C6 hydroxyalkyl with the hydroxyl group in position 2 relative to the quaternary nitrogen or R
1
;
R
4
is hydrogen, methyl, C2-C10 alkyl, C3-C8 cycloalkyl or C2-C12 alkoxy;
R
5
is C1-C4 alkyl;
Y
−
is R
6
COO
−
or OH
−
; and
R
6
is hydrogen or a branched or unbranched, aliphatic or araliphatic, C1-C10 alkyl radical.
Another embodiment of the invention provides a trimerization catalyst of the formula (I):
wherein
and where the variables are defined as follows:
A, B, C, D and E independently of one another or simultaneously are hydrogen, chloro, C1-C4 alkyl, C1-C4 alkoxy, C1-C4 alkoxycarbonyl, hydroxyl, (R
5
)
3
SiO—, (R
5
)
2
N—, —COOH, (R
5
)
2
N—CH
2
— or phenyl, it being possible for any two adjacent A, B, C, D and E radicals to form a conjoint 5- or 6-membered saturated or unsaturated ring which may optionally and additionally include N, S or O as heteroatom;
F is hydrogen or methyl;
R
2
and R
3
independently of one another or simultaneously are C1-C4 alkyl, C2-C6 hydroxyalkyl with a hydroxyl group in position 2 relative to the quaternary nitrogen in formula (I), or R
1
;
R
4
is hydrogen, methyl, C2-C10 alkyl, C3-C8 cycloalkyl or C2-C12 alkoxy;
R
5
is C1-C4 alkyl;
Y
−
is R
6
COO
−
or OH
−
; and
R
6
is hydrogen or a branched or unbranched, aliphatic or araliphatic, C1-C10 alkyl radical.
DETAILED DESCRIPTION OF THE INVENTION
Various other objects, features and attendant advantages of the present invention will be more fully appre

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